The present invention relates to apparatus for the development of electrostatic latent images formed on a photo-sensitive drum, and more particularly, to such apparatus which uses a microcapsule developing agent.
A conventional electronic photo copying machine is shown in FIG. 1. A photo-conductive drum (electronic latent image carrier) 1 includes an aluminum substrate and a photo-conductive layer formed thereon. The surface of the drum 1 is uniformly electrostatically charged by an electrifying unit 23 prior to irradiation with a picture image. An original mount carriage 11 on which an original 13, e.g., a book to be copied, is mounted is disposed above the photo-conductive drum 1. Below the original mount carriage 11 there are disposed a light source 12 adapted to emit light against the surface 13 of the original to be copied and an optical system 14 adapted to direct light reflected from the original 13 toward the photoconductive drum 1.
An electro-static latent image is formed on the surface of the drum 1. This latent image is developed by use of a developing unit 15 disposed adjacent the surface of the drum 1 to develop an image, using a developing agent (or toner), on the drum's surface. This image is moved, by rotation of the drum, to an image transfer unit 16 at which location the toner image is transferred or copied onto a sheet 18 supplied from a sheet feed unit 17.
The sheet 18 carrying the transferred toner image is subjected to electron removal at an electrifying unit 19 and the sheet is peeled from the drum surface. The peeled sheet 18 is fed to an image fixing unit 20 where the toner image is fixed onto the sheet 18.
Residual toner on the surface of drum 1, which has not been transferred onto the sheet 18, is deelectronized by a precleaning corotron 21 after the transferring step, and then removed from the surface of the drum 1 by a cleaning unit 22. The cleaned surface is then in the stand-by state for a subsequent copying cycle.
The methods for fixing the toner image onto the sheet 18 are generally classified into a thermal-fixing method, a solvent fixing method, and a pressure fixing method.
The pressure fixing method has certain advantages in that with the use of this method the electric power consumption can be minimized, since a heat source, which is required in the thermal-fixing method, is not required. Further, the pressure fixing method requires only a short access time for initiating the copying, since it is unnecessary to wait for the temperature to be raised to a given temperature which is required in the thermal-fixing method.
The pressure fixing method conventionally uses a developing agent which contains wax as a primary component. However, such a developing agent does not provide sufficient fixing force relative to the paper. Therefore, there is a disadvantage that, if the paper is folded or rubbed, the picture image may be peeled from the sheet. In order to solve this drawback, a microcapsule developing agent has been proposed in which a core substance having an excellent image fixing property is confined in an outer shell which is rupturable under pressure to expose the core substance. While such a microcapsule developer particle has sufficient fixing capability its rupture strength is excessively low and, in fact, is much lower than that of a conventional developer agent particle, including ordinary compositions such as the combination of resin and carbon black.
Accordingly, if such microcapsule developer particles are used in a conventional two component developing system, which uses a toner and a carrier, the microcapsule particle is readily ruptured due to pressure contact with the carrier, and a stabilized image may not be obtainable.
Therefore, a single component microcapsule developing agent using only a magnetized toner comprising a magnetized powder has been proposed.
A conventional photo copying machine which uses a single component such magnetized toner as a developing agent is shown in FIG. 2. FIG. 2 shows a photo sensitive drum 1 adapted to carry the electro-static latent image 2 thereon which is positioned adjacent and opposite a developing unit D. The developing unit D includes a hopper 3 for containing a supply of single component magnetized toner 4, an agitator (not shown) for agitating and feeding the toner 4, a magnetic roll 5, a cylindrical non-magnetic sleeve 6, and a toner regulation member 7. The stationary magnetic roll 5 provides a plurality of magnetic poles N and S alternatingly arranged circumferentially around the roll 5. The sleeve 6 is rotatably supported and disposed over the magnetic roll 5. The regulation member 7 resiliently contacts the outer peripheral surface of the sleeve 6 and regulates the amount of the toner deposited onto the sleeve 6. The sleeve 6 and the regulation member 7 are disposed in an internal space of the hopper 3. Further, the sleeve 6 is disposed in the vicinity of the photo-sensitive drum 1 at the developing region A.
In the operation of such developing apparatus, a controlled amount of toner is deposited on the outer surface of sleeve 6 by the regulation member 7, and the single component magnetic toner 4 is held on the surface of the sleeve 6 by the magnetic force of the magnet roll 5,. Upon electrification, the toner 4 is fed, by the rotation of the sleeve 6, into the developing region A where the sleeve 6 and the drum 1 are opposite each other. Therefore, the sleeve 6 serves as a carrier member for carrying the toner. The sleeve 6 is connected to an alternating current source 8 and a direct current source 9, so that AC-DC superimposition voltage is applied to the sleeve 6. Toner adhered onto the sleeve 6 is electro-statically attracted to the electro-static latent image 2 formed on the surface of drum 1 when it rotates into the developing region A, whereupon it is developed for visualization.
The particles of the single component magnetizable toners 4 may become magnetically bunched up together, and form a soft mass, or compacted powder, during rotational movement of the toner together with the rotation of the sleeve 6. During repeated motion of the toner, the toner particles may become relaxed on the sleeve surface and are returned to the hopper 3 because of magnetic repulsion. Also, the particles are attracted to the sleeve surface by the magnetic force of the magnet. The soft powder mass or compacted powder may be transported through an area of contact between the regulation member 7 and the sleeve 6 whereupon the powder mass is subjected to pressure. In this case, if ordinary magnetizable toner consisting of resin and magnetic powder is used, the soft powder mass is ruptured because of the contact pressure between the sleeve 6 and the regulation member 7, so that the toner layer has a uniform thickness along the peripheral surface of the sleeve 6.
On the other hand, if a microcapsule toner is used in the device shown in FIG. 2, which is particularly adapted for the use of a single component developing agent, the capsules may be ruptured by the contact pressure, due to the low strength of the capsule. Therefore, the core substance may flow out of the capsule, so that the core substance may adhere to adjoining particles and thereby create a rigid powder mass that degrades the uniformity in thickness of the toner layer on the sleeve 6.